Chapter on the Prayer Times (Mawaqit)
Tahdhīb al-Aḥkām|Volume 2|Book 1|Chapter 13
اَلْحَسَنُ بْنُ مُحَمَّدِ بْنِ سَمَاعَةَ عَنِ اِبْنِ مُسْكَانَ عَنْ زُرَارَةَ عَنْ أَبِي جَعْفَرٍ عَلَيْهِ اَلسَّلاَمُ قَالَ: أَ تَدْرِي لِمَ جُعِلَ اَلذِّرَاعُ وَ اَلذِّرَاعَانِ قُلْتُ لِمَ قَالَ« لِمَكَانِ اَلْفَرِيضَةِ لَكَ أَنْ تَتَنَفَّلَ مِنْ زَوَالِ اَلشَّمْسِ إِلَى أَنْ يَبْلُغَ ذِرَاعاً فَإِذَا بَلَغَ ذِرَاعاً بَدَأْتَ بِالْفَرِيضَةِ وَ تَرَكْتَ اَلنَّافِلَةَ»
Al-Hasan ibn Muhammad ibn Sama‘ah from Ibn Miskan from Zurarah:
Abu Ja‘far عليه السلام, peace be upon him, said: "Do you know why the arm's span (dhira‘) and two arm's spans were set?" I asked: "Why?" Imam عليه السلام said: "Because of the obligatory prayer. You are allowed to perform supererogatory (nafilah) prayers from the sun's zenith until the shadow reaches an arm's span. When it reaches an arm's span, you must begin with the obligatory (faridah) prayer and leave the supererogatory."
- Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 2, p. 262
- Ṣaḥīḥ al-Tahdhīb, p. 157